III. Boron Availability in Soils

present in them to be partially readily available. They did not believe

that this test proved conclusively the availability of boron in tourmaline,

however, because of the presence of other materials.

Berger and Truog (1940) ground pure tourmaline crystals to pass a

100-mesh screen and found that even with the unwashed material, sunflowers could not obtain boron from it at a fast enough rate to continue

growth.

From these results it seems plausible that in the humid regions the

available boron in the soil may be in a form other than tourmaline, but

that tourmaline, as i t gradually decomposes, replenishes the supply of

available boron. If leaching is very rapid in acid soils or if large amounts

of lime are added so as to cause fixation, or crop removal is great, the

replenishment from tourmaline will not be fast enough to prevent boron

deficiency in crops with a high requirement. If, on the other hand, conditions favoring availability of boron are good, the amount of boron supplied by tourmaline will probably be adequate for normal growth of crops.

of California the subsoils of which a t a depth of 2 to 3 feet contain as

much as 4.5 p.p.m. of available boron. Purvis and Hanna (1938), in a

field experiment, found that applications of boron did not injure a second

crop of snapbeans planted three months after a first crop had been

planted and injured-by an application of 50 lbs. of borax per acre. A

similar experiment in pots resulted in injury to both crops, showing that

the boron was leached in the field and not fixed.

c. Texture. Although there has not been much work done on correlations between available boron in soils and texture, usually in the humid

regions the lighter textured soils contain less available boron in the plow

layer than do slightly acid, silt and clay loams. Lehr (1940) has shown

that clays, particularly those of marine origin, are high in boron and in

sands are low. He estimates the amounts of total boron in the various

types of Dutch soils as follows: marine clays about 100 p.p.m. boron;

river clays about 20 p.p.m. boron; sandy soils average about 6 to 25

p.p.m. boron in tourmaline, and 1 to 2 p.p.m. boron in organic matter.

Kubota et al. (1948) also show that sandy soils contain less available

boron t.han do soils with heavier textures.

The effect of texture, however, has much less influence on the amount

of available boron than does organic matter or pH.

2. Conditions Favoring Fixation or Loss

It has been known for a long time that boron deficiencies occur

frequently on alkaline soils in humid regions. The natural assumption

330

K. C. BERGER

was to attribute this to over-liming. It was suggested by Naftel (1938)

that the boron deficiency as a result of over-liming may be due to absorption of the boron by the increased population of soil microorganisms,

which might accompany the change in reaction caused by liming. It is

quite generally accepted now that this is not correct because of the speed

with which reaction takes place. A number of conditions favor fixation

in, and loss of boron, from soils.

a. Alkalinity and Change in p H . One of the first indications that

boron is fixed in soils in a form unavailable to plants was obtained by

Bobko and Syvorotkin (1935) when they noticed that small additions of

boron to soils counteracted the harmful effect of overliming. Naftel

(1937) obtained similar results. Wolf (1940) added sodium, potassium

and magnesium hydroxides to a soil of p H 5.9. He found t,hat magnesium causes the greatest reduction in availability of soil boron, with

calcium, sodium, and potassium hydroxides having lesser effects in the

order named. He based his conclusions on the amount of boron found

in the roots and tops of radishes grown on the soil. The work is subject

to crit,icism because of the upset physiological condition of the plants due

to the wide variation in calcium-magnesium ratios, caused by the addition of large amounts of magnesium hydroxide. Tulin (1940) found that

reduction in plant yield due to the addition of calcium carbonates was

reduced by the addition of magnesium carbonates, probably due to bringing about a more favorable calcium-magnesium ratio.

Olson and Berger (1946), investigating the effect of alkalinity on

boron fixation, found that the effect of boron fixation in soils is closely

related to the clay cont,ent and soil reaction as changed by the addition

of sodium hydroxide, calcium hydroxide, or hydrochloric acid. The

cations of the bases had little influence on the boron fixation but the

alkalinity produced by them resulted in fixation. With a prairie soil,

calcium hydroxide was more effective in inducing boron fixation than was

sodium hydroxide, while with a wooded soil, Spencer silt loam, the bases

were equally effective. Even a t pH values of 9.5 or higher, however, not

more than 40 per cent of the added or native available boron in the soil

was fixed.

Olson and Berger also found that upon oxidation of soil organic matter, there was a slight decrease in boron fixation, and that of the inorganic

fractions of the soil, the clay separate fixed the greatest amount of boron.

Midgley and Dunklee (1939, 1940) and Dunklee and Midgley (1943)

found that very large amounts of boron were fixed by soils when the pH

was changed greatly. Acid leaching, and then liming to neutrality,

caused boron fixation up to 90 per cent of the boron added. This was up

BORON IN SOILS AND CROPS

331

to four times as niiich fixation as on the unlimed soils. They state that

both the organic and inorganic fractfixation.

Parks (1944), using chemical tests, found that thc addit,ion of lime

decreased the amount of boron fixed during the drying of a Dunkirk fine

sandy loam. He concluded that the effect of overliming in causing boron

deficiency in plants was entirely physiological and not due to a decrease

in boron availability in soils. Chemical tests of soil extracts show that

this view is not entirely correct. Colwell and Cummings (1944) state

that potassium and sodium metaborates form small units and crystals

whereas calcium metaborates form an endless chain structure. They

determined the ionic conductance of borate solutions and found the conductance of calcium metaborates lower than that of the sodium or potassium salt, indicating a polymerization of calcium metaborate molecules.

They suggested that such a polymerization of calcium metaborates would

cause a decrease in the uptake of boron by plants. The high solubility

and availability of calcium metaborates to plants in culture solutions,

however, indicates that factors other than this are responsible for boron

fixation.

Parks and Shaw (1941) in an experiment with pure chemicals precipitatcd boron in combination with silica and aluminum. The presence

of calcium ions, drying, and high pH, all tended to increase the boron

content of precipitates. They believed that the data indicated that boron

fixation might be due to small amounts of boron entering into complexes

of calcium with silica or aluminum or into calcium aluminosilicate

products of synthesis as a substitution product for aluminum ions. This

and the calcium saturated bentonites were the only two systems studied

in which boron in the precipitates a t p H 8.0 was insoluble in boiling

distilled water. It seems highly probable that the fixation of boron in

this system might be due to occlusion. Boron fixed under alkaline conditions can again be released by acidification of t.he soil (Olson and Berger,

I-3-c found that more boron was obtained from an acid extraction of the

soil than from a water extraction. Berger and Truog (1940) also arrived

at the same conclusion. Haas (1944b) demonstrated that sulfur appljcation to the soil in the field increased the water-soluble boron content of

the soil from 1.99 p.p.m. to 4.26 p.p.m. in the top foot. I n the light of

this evidence it appears probable that boron does not precipitate out in

the form of insoluble silicates.

It seems apparent that boron is fixed largely in alkaline soils,

in t.he presence of free calcium, partially by organic matter and partially

by the soil minerals. I n these forms i t is temporarily unavailable to

332

K. C. BERGER

plants but can be released by the action of strong mineral acids and also

probably is released by the decomposition of organic matter.

b . Calcium-Boron Ratios. Since Brenchley and Warington (1927)

first indicated that there was an association between boron and calcium

absorption by plants, numerous workers have studied boron-calcium

ratios in plants and the interelationships of various other element8 with

boron. Marsh and Shive (1941), working with corn, found that the

metabolically effective calcium which is maintained in the soluble state

in the active plant tissues is directly correlated with the supply of available boron in these tissues. Later, Reeve and Shive (1944) found that as

the calcium content of a nutrient solution was increased, more boron was

required to prevent boron deficiency in the plant, and that more boron

could be added to the solution without the development of symptoms of

boron toxicity.

Jones and Scarseth (1944), working with a number of Indiana farm

crops, in greenhouse experiments on limed and unlimed soils, found t,hat

plants would take up varying quantities of calcium and boron depending

upon the availability of these elements in the soil. Analysis of the plant

showed that normal growth would occur only when a certain balance in

the intake of calcium and boron existed. They found low tolerance for

boron with low calcium in the plant, and a high requirement for boron

with high calcium. Drake et al. (1941) have found that growth of Turkish tobacco on a Norfolk sand appeared normal when the calcium-boron

ratio in t,he plant did not exceed 1340:l. A calcium-boron ratio of

1500:l in the plants was correlated with severe boron deficiency.

From these data it is obvious that not only is boron fixed in alkaline

soils in a form temporarily unavailable to plants but that more boron

is required by plants growing on these high calcium soils.

c. Leaching. Another factor that contributes to the loss of boron in

some soils, and consequently contribut,ing to deficiency, is that of leaching. One of the first field investigations on boron movement was conducted in New Zealand by Askew and Thomson (1937) and Askew et al.

(1938), who found that boron applied to an apple orchard 2 years previously had moved to a depth of 30 inches in this period. A similar study

by Woodbridge (1940) indicated that greatest movement was found in

the lighter soils 2 years after application and the least movement in the

of New Jersey to soils to which borax had been applied a t the rate of

20 lbs. per acre. They found that about 85 per cent of the boron leached

from a sandy soil and 75 per cent of the applied boron leached from loam

and silt loam soils.

I n a comprehensive study of Wisconsin soils, Kubota et al. (1948),

working in the field and laboratory, found that the rate of boron movement was related primarily to the soil texture. Where the soil was uniformly light-textured throughout the profile, much of the applied boron

moved to a depth of 24 inches or deeper in 6 months. I n the heavier

soils, little of the boron moved below the Winch layer. In laboratory

experiments it was found that the bulk of applied boron moved downward in mass rather than in portions. The movement of boron was found

to lag behind the movement of the water, and was also found to be independent of the movement of sodium in the borax. Following application

of borax and leaching with 2 inches of water, most of the sodium was

found to be in exchangeable form in the surface 3 inches of a sand and

silt loam soil, while the boron was found at greater depths. Liming

reduced the rate of boron movement. Calcium applied as calcium

chloride decreased the rate, but less than did lime.

From these studies it can be seen that leaching, particularly in acid

soils, is a very imporbant factor in the loss of boron from soils.

d. Drying. Drying has been shown by Parks (1944) and others t o

cause increased fixation of boron in the laboratory. Olson (1947) found

that the drying of soil after boron was added increased the amount of

boron fixed. The increase was greater in the case of limed soils than

with unlimed soils Drying at 60°C. increased fixation more than drying

a t 20°C. These findings confirm those of Parks, who found that drying

a t 85°C. almost doubled the amount of boron fixed when soil was dried

a t 26°C. Walker et al. (1944) observed that boron deficiencies were

more severe in places in the fields where soil dried out excessively in dry

years. Latimer (1941) showed that drought in June and July was the

most important factor in the cause of internal cork in New Hampshire

apple orchards.

It is doubtful whether the plow layer in the field will dry out enough

in most years so as to cause appreciable boron fixation. I n the first place,

334

*

K . C. BERGER

if boron was fixed to any extent, most soils would be very deficient in

boron particularly those in the arid regions. This boron would not be

released in available form in wet years. This is obviously not the case.

Rather, it is suggested that the reason for more boron deficiency in dry

years than in wet years is that, as has been previously shown in Section

II-1-a, most of the available boron is found in the surface organic layer.

When this layer becomes relatively dry, plants feed in it but little.

Thus it is necessary that they feed off the lower horizons of the soil which

are usually low in available boron and organic matter. This causes boron

deficiency in crops during dry years because their supply of available

boron has been reduced, not so much by fixation, but by lack of ability

of the plant roots to feed in the surface horizon because of a lack of

water. It is quite possible that there is some boron fixation due to drying

in surface soils in extremely hot and dry weather, but is very doubtful

if much boron is fixed below the 2-inch depth.

e. Crop Removal. Another way in which boron is lost from soils is

by crop removal. Because boron is found in such small quantities in

plank, this is not often considered to be a serious loss. Reeve et a!.

(1944), however, have shown that, in New Jersey, alfalfa hay contained

boron equivalent to nearly 2 lbs. of borax per ton when grown on soils

that were adequately supplied with the element. I n most humid region

soils, however, the boron removal from soils by alfalfa is much less than

this. Crop removal is a factor that is important and this is evidenced

by the increasing number of areas in the world in which boron deficiency

has been observed.

3. Boron Cycle

A cycle of boron in nature has been diagrammed by Dennis (1937).

This cycle implies the permanent removal of boron into coal formations,

iron and manganese ores, and borosilicate minerals. This cycle is not

detailed as far as soils and plants go, because of lack of information

when it was proposed.

A more detailed cycle of boron in humid region soils is given in

Fig. 1. This cycle is a summary of Section 11, of this article.

As can be seen, boron is removed from the soil by leaching, and by

removal in plants. Boron is added to the soil by fertilization with boron

fertilizers and by decomposition of tourmaline and small amounts of

other primary soil boron minerals. The available soil boron is in two

forms: organic and inorganic. These are in equilibrium with each other

and with unavttilable organic and inorganic soil boron. Available boron

enters the plants during growth, and again appears in an available form

BORON IN SOILS AND CROPS

335

boron

Microbial

growth

followed

by death

Decay

t o form

calcium

borate

Boron fertilizers

Temporarily

free Ca

boron

acidity

!

Leaching

Tourmaline and small

amounts of other primary

soil boron minerals

I n acid soils, much of the boron remains in the inorganic form and

is leached out. This cau8es acid sandy soils to be low in available boron.

In general in the humid region, soils low in available boron are lightcolored acid soils and alkaline soils regardless of the organic matter

content. I n arid and semi-arid regions, because of the absence of leaching, quantities of inorganic soil boron occur probably as sodium and

calcium salts.

336

K. C. BERGER

IV. BORON

REQUIREMENT

OF PLANTS

As was stated in the introduction, there has been a tremendous amount

of work on boron in soils and plants, and since the first field proof of

boron deficiency was given shortly after 1930, nearly 2000 papers on boron

have been published. The major part of these have dealt with the boron

requirement of plants. Shive (1945) gave an excellent historical survey

of boron in plant life in which he pointed out that, although Wittstein

and Apoiger discovered boron in plant tissue in 1857, the significance

of the discovery was not realized until years later when four different

men between 1888 and 1890 detected boron in wines and certain fruits

and in leaves and tissues of a great variety of plants. Finally Jay

(1895), in a rather comprehensive investigation, concluded that boric

acid is generally dist%ributedthrough the earth’s crust, that both cultured

and wild plants take up boric acid from the soil and from water, and that

when boric acid is introduced in the stomachs of animals it is not assimilated but is nearly all excreted. Jay further stated that boron occurs

universally in autotrophic plants, and is known to be distributed in at

least some of the heterophytes such as mushrooms. It was also discovered that plants differ widely in their ability to absorb boron from

growth of peas and spinach as a result of adding boron to soil. I n 1910,

Agulhon obtained increased yield of wheat, oats, and turnips by the use

of boron in nutrient solutions. He suggested that the boron is a useful

element for higher plants and included it with manganese as an element

occurring only in very minute amounts, and exercising a function which

he regarded as catalytic. Maz@ (1915) was the first to demonstrate

that boron was essential for the normal growth of the corn plant. By

the use of new and greatly improved met.hods of experimentation, he

produced evidence which led him t o the conclusion that boron was essential for corn. He also concluded that other elements not previously

considered essential might be found indispensable for green plants. Warington (1923) showed that boron was indispensable for the broadbean and

that in the absence of boron, this plant did not complete its life cycle

but died prematurely with characteristic symptoms. Preliminary results

wit.h other species such as barley, crimson clover and several species of

beans lead to the general belief that boron is essential to these species

also, but the evidence was held to be inconclusive.

BORON IN SOILS AND CROPS

337

Stiles (1946) listed the name of the first worker to call definite attention to the favorable effect of boron on the growth of the species concerned. This was given whether the worker regarded boron as essential

for the species or not. Thus, Nakamura in 1903, reported increased

growth of peas and spinach as a result of adding boron to the soil but

i t was not until 1915 that M a d claimed the essential nature of boron

for plant growth.

1, Function of Boron in Plants

Warington (1923) first showed that meristematic activity was markedly affected in the broadbean and that both roots and stem tissues were

abnormal in the absence of boron. Since this work, there have been a

great number of studies on the function of boron in plants. Johnston

and Dore (1929) found t.hat plants grown in a boron deficient nutrient

solution showed four distinct types of injury: (1) death of the terminal

growing point of the stem; (2) breakdown of the conducting tissues of

the stem; (3) a characteristic brittleness of the stem and petiole and

(4)extremely poor growth of roots which develop a brownish unhealthy

color. The total sugars and starches were more abundant in the leaves

and stems of the boron deficient plants while a greater amount of benzene-insoluble matter was found in the leaves of normal plants and in

the stems of boron deficient plants.

Haas and Klotz (1931) concluded that boron is essential to cell division in the meristematic tissues and in the cambium. In the absence

of sufficient boron the cambium and portions of the phloem were observed

to disintegrate and gum up, some of which found its way to the exterior

through a split in the cortex. When there was any xylem disintegration,

the amount was small. A normal accumulation of carbohydrates in the

leaves of boron deficient, plants was observed and ascribed to the disintegration of the phloem with consequent interference with translocation. The addition of boron to the culture solution resulted in a reduction

in the total sugar content of the leaves and a restoration of the vigor

of the plant.

Shive (1941) believed there was considerable experimental evidence

that boron is an important factor in the processes involved in organic

synt.hesis. He found that plants grown in boron deficient regions yielded

strong positive tests for pectins and negative tests for fats. Lohnis

(1940), studying the influence of boron deficiency on the anthers of

several small grains, found the primary effect of boron deficiency to appear in the cell nucleus where division was inhibited in the early stages